Is this for trailers with a swan neck? We don't see those over here (UK) but it's a better option for towing a trailer than a hook bolted to the chassis/body.
Do they apply weight directly to the back axle like a proper truck's fifth wheel coupling are are they just bolted to the bed?
EDIT: Dually as in dual rear wheels. I get it now.
Stevce.
Just like a lorry (or, as we call them in the States, a semi*), but smaller and without the air brakes. Usually the trailer brakes are electric. The brake controller is activated by a tap from the brake/stop light circuit and an inertial sensor is used to adjust the braking force.
There are typically two different attachment methods: a "5th wheel" hitch where a vertical pin on the tongue of the trailer slides into the side of a disc and is locked in place (just like a lorry), or a "ball" style hitch -- both in the bed of the pickup over the rear axle.
*"Semi" is short for "semi-tractor trailer rig." Attached image shamelessly stolen from trucktrend.com